The present invention relates to a swashplate type compressor for compressing a gas. More specifically, the present invention relates to an enriched cobaltxe2x80x94tin coating alloy that is particularly well suited for providing a coating surface on an aluminum or aluminum alloy swashplate of such a compressor. The present invention also provides a method of coating a swashplate with an enriched cobaltxe2x80x94tin coating alloy.
Swashplate type compressors are frequently used in refrigeration circuits such as those used in automotive applications. These compressors operate through an engagement between one or more pistons and a swashplate. Rotation of the swashplate; which is oriented at an angle to the axis of rotation of a drive shaft, causes the engaged pistons to reciprocate within individual cylinder bores. This reciprocation allows the pistons to compress the refrigerant as part of the mechanical refrigeration process.
A pair of shoes are disposed between each piston and swashplate and swivel within concave pockets as the swashplate rotates. The interface between the shoes and swashplate is an area of sliding contact, giving rise to friction and wear. Over time, wear can lead to seizure of the compressor, especially in conditions in which lubricant supply to the swashplate is inadequate. This failure mode is one principal limitation on the useful life of a swashplate type compressor. Problems of maintaining low friction sliding contact between the shoes and swashplate are especially challenging for swashplates made of aluminum and aluminum alloys, because these materials are relatively soft, yet have abrasive qualities.
The present invention provides a swashplate-type compressor having a coating disposed on the aluminum or aluminum alloy swashplate. The coating preferably comprises a multi-layered structure on the surface of the swashplate substrate. Preferably, the coating is placed on the substrate in at least the areas that come into contact with the shoes. In a preferred embodiment, a base layer of tin is disposed on the substrate, and a layer of cobalt-tin is disposed on the tin layer. The cobalt-tin layer comprises between 12 and 28 weight % cobalt, with the balance comprising tin. In a preferred embodiment, the cobalt-tin layer comprises about 22 weight % cobalt, with the balance comprising tin.
The present invention provides a high weight percentage of cobalt in the surface coating layer. These layers can be deposited by chemical or electroplating deposition processes. However, electroplating is preferred since it allows for the achievement of higher cobalt concentrations, which improves the seizure resistance of the swashplate and compressor.
Electroplating deposition processes, when used in connection with this invention, also allows for the elimination of a second polishing step normally required for chemical deposition, commonly referred to as xe2x80x9csuperfinishingxe2x80x9d. This step is normally required to achieve a desired chemically reactive surface of the swashplate and is done immediately prior to chemical coating. The enriched-cobalt-tin alloy according to the present invention provides a desirable coating that can be electroplated onto a machined surface. Such surfaces can be obtained in a single machining operation, and thus superfinishing can be eliminated. Consequently, the coating alloy and method of the present invention when applied by electroplating offers significant time and expense savings over coatings deposited by chemical conversion techniques.